Electrostatic atomizers disperse liquid by applying a net electrical charge to the liquid, typically as a stream of the liquid passes through an orifice. The negative charges developed within the liquid tend to repel one another, dispersing the liquid into droplets. The injection of the net charge into the liquid may be accomplished utilizing a pair of opposed electrodes arranged adjacent to the stream of liquid and electrically connected to a high voltage power source. Such an electrostatic atomizer, called the SPRAY TRIODE.TM. atomizer, is disclosed in certain embodiments of U.S. Pat. No. 4,255,777, the disclosure of which is hereby incorporated by reference herein. Another electrostatic atomizer utilizes an electron beam to apply a net negative charge to the liquid. Certain embodiments of U.S. Pat. Nos. 5,093,602 and 5,378,957, the disclosures of which are hereby incorporated by reference herein, disclose apparatus and methods for electrostatic atomization utilizing an electron beam.
Electrostatic atomization of Newtonian fluids adheres to the following equation: D=75/.rho..sub.e. D is the mean droplet size in microns and .rho..sub.e is the charge density of the fluid, in coulombs per meter cubed. Thus, the same size droplets will be produced whenever a particular charge density is achieved.
The greater the charge density injected into the liquid, the greater the droplet dispersion, the smaller the droplet size and the narrower the droplet distribution. A limit on the charge density which can be injected into the liquid is the phenomenon of corona-induced breakdown, which interrupts dispersion of the liquid. When a critical level of charge is reached, the spray plume collapses. FIG. 7A shows a spray plume during uninterrupted operation and FIG. 7B shows a spray plume during operation interrupted by corona-induced breakdown. For a combustion device, this means interruption of the flame operating on the electrostatically atomized fuel.
For example, a combustion device has been run on fuel atomized by the SPRAY TRIODE.TM. electrostatic atomizer. It was found that sustained operation close, i.e., within 50V, to the critical level for corona-induced breakdown, which was about 5 kV or more, was required for blue flame operation. However, when the net charge reached the critical level, operation of the combustion device was dramatically interrupted. Furthermore, the critical level of net charge at which corona-induced breakdown occurs depends upon the properties and flow rate of the fuel, which vary during operation of the combustion system. Changes in ambient pressure and temperature also affect the operation of the electrostatic atomizer.
It would be desirable to develop an electrostatic atomizer with improvements in sustained operation and the maximum charge density provided to a liquid.